Static converter arrangement

ABSTRACT

A static converter arrangement comprises at least two converter units having substantially the same configuration and each of which includes at least two series-connected semi-controlled converter bridge circuits. At least two of such bridge circuits of one converter unit have an uncontrolled bridge portion in common and are connected to a tapped secondary winding of a transformer such that one top is connected to a circuit point between anode and cathode of the semi-conducter valves of the uncontrolled bridge portion via a choke. Thus the converter arrangement comprises at least two chokes, one for each connector unit, and these chokes are magnetically in reverse-series connection.

United States Patent 91 Winter Jan. 29, 1974 [73] Assignee: Brown Boveri& Company Limited, Baden, Switzerland [22] Filed: Sept. 19, 1972 [21]Appl. No.: 290,410

[30] Foreign Application Priority Data Oct. 11, 1971 Switzerland14712/71 [52] US. Cl 307/17, 318/61, 318/64, 318/91, 318/94, 318/109,321/47 [51] Int. Cl. 1102p 7/68, H02p 13/24 [58] Field of Search 307/17,33; 318/67, 98, 99, 318/341, 345, 61, 64, 91,94, 109; 321/26,

4/1965 Berman 318/345 X 12/1970 Smith 318/345 X Primary ExaminerA. D.Pellinen Attorney, Agent, or Firm-Ralph E. Parker et a1.

[ 5 7] ABSTRACT A static converter arrangement comprises at least twoconverter units having substantially the same configuration and each ofwhich includes at least two seriesconnected semi-controlled converterbridge circuits. At least two of such bridge circuits of one converterunit have an uncontrolled bridge portion in common and are connected toa tapped secondary winding of a transformer such that one top isconnected to a circuit point between anode and cathode of thesemiconducter valves of the uncontrolled bridge portion via a choke.Thus the converter arrangement comprises at least two chokes, one foreach connector unit, and these chokes are magnetically in reverseseriesconnection.

3 Claims, 3 Drawing Figures STATIC CONVERTER ARRANGEMENT SUMMARY OF THEINVENTION A major problem with static converters is that when theirignition phase angle is not fully advanced for partial load in thechopped-wave control operation they cause severe harmonics in thenetwork current. The use of chokes to reduce the harmonics is known. Anunfavourable consequence of these inductances is generally that thecommutation of current is delayed with respect to voltage zero. A phasedisplacement between the alternating current and voltage, however, isequivalent to a greater reactive power requirement.

It has been proposed already to provide the secondary winding of thetransformer of the converter with a central tapping, a choke beingconnnected in the line leading away from the central tapping. The chokethen acts beneficially in that it is effective only when the converteris at partial load (reduction of harmonics), but not when it is at fullload.

If in an arrangement with semi-controlled converter bridges one of thesebridges subjected to the lowest a.c. voltage is operated alone at fullload, the choke similarly delays the current rise. Even without a chokethe network current would rise sufficiently slowly because of thevoltage half-wave rising from zero (no haromonics at full load), and theresult would therefore be an unnecessary worsening of the power factor.

The object of the invention is to create a converter arrangement whichcauses few harmonics in the network current and exhibits a better powerfactor. This is achieved in that the chokes of the converter units arearranged magnetically in reverseseries connection.

Particularly in the case of a.c. traction vehicles (locomotives), atleast two such converter units of the kind stated above are combined toform one converter arrangement. With two, three or four bogies (driveunits) a separate converter unit is allocated to each bogie.

Concerning a first version, in which the secondary windings of thetransformer are provided exclusively with a central tapping, it isrecommended that the chokes should be connected in the lines leadingaway from said central tappings.

Concerning a second version, in which the secondary windings referred toare provided with tappings after one-fourth and one-half of their totalnumber of turns, it is preferable that the chokes should be connected inthe lines leaving after one-fourth of the total turns.

The advantage of the invention lies particularly in a significantimprovement of power factor. Furthermore, a considerable saving ofweight and space is achieved in that the respective choke coils of theconverter units are located on a common core. Although the size of thecore must be suited to the number and dimensions of the chokes, asregards the magnetic stress it is necessary to make allowance only forthe magnetic flux expected to be caused by a single choke. If two, fouror six chokes are carrying current simultaneously, the resultant flux iszero.

DETAILED DESCRIPTION OF THE INVENTION The invention will now beexplained in more detail with reference to the drawings, in which:-

FIG. 1 shows diagrammatically an example of a first version of theinvention;

FIG. 2a shows a voltage/time diagram;

FIG. 2b shows another voltage/time diagram;

FIG. 20 shows a current/time diagram;

FIG. 3 shows an example of a further version of the invention.

FIG. 1 shows two converter units I and II, each comprising twosemi-controlled converter bridge circuits connected in series on thea.c. side, being connected to two secondary windings 6, and 6,, of atransformer. The primary winding I of the transformer (taking theexample of a locomotive) is connected to a single-phase a.c. network, onthe one hand via a current collector 2 and a conductor wire 3, and onthe other via (at least) one wheel 4 and a rail 5.

Each converter unit feeds a motor 13, or 13 motor 13, for example,feeding the foreward driving wheels, when viewed in the direction oftravel. The mode of operation of converter unit I will now be explained,this being representative of both units.

In order to vary the d.c. output voltage continuously up to the maximumvalue, thyristors 7, and 8, being provided as controlled valves arefirst triggered in known manner, i.e., 7, during one half-wave and 8,during the other, the thyristors initially being fired in such a waythat they conduct for only brief periods, and thus chop only smallsections out of the a.c. network voltage (ignition phase-angle control).To increase the d.c. output voltage the thyristors are firedprogressively earlier until at full load thyristor 7,, for example,conducts the complete positive half-wave, and 8, the following negativehalf-wave.

The controllable valves, i.e., thyristors 7, and 8, together with theuncontrollable valves, i.e., diodes 9, and 10, thus form onesemi-controlled converter bridge circuit, and thyristors 11, and 12,with the same diodes 9, and 10, form a second such semi-controlledconverter bridge circuit.

While the ignition phase angle of thyristors 7, and 8, is beingadvanced, the direct current flows during the positive voltagehalf-wave, for example, from the central tapping of the secondarywinding 6, via a choke 14,, diode 10,, motor 13, and thyristor 7, backinto the lower half of secondary winding 6,. During the negativehalf-wave the direct current commutates from thyristor 7, to diode 9,,then flows in the opposite direction through choke 14,, by way of thecentral tapping through the lower half of secondary winding 6, andthyristor 8, to the motor 13,.

When thyristors 7, and 8, are fully conducting, the d.c. voltage outputavailable is 50 percent of the possible maximum because only the lowerhalf of secondary winding 6, is effective.

Once full load is reached, thyristor 12,, for example, can take over inplace of thyristor 7,, and thyristor ll takes over from thyristor 8,,whereupon the upper half of the secondary winding 6, becomes effective.

To further increase the d.c. output voltage, thyristors 7, and 8, areagain operated in the chopped-wave mode, while thyristors l1, and 12,remain at full load. This form of control has the advantage that thecontrol device of only one converter bridge circuit (thyristors 7,, 8,and diodes 9,, 10,) has to be capable of ignition phase-angle control,while a simple control device is sufficient for the second bridgeseries-connected on the a.c. side (thyristors 11,, 12, and diodes 9,,10,).

FIGS. 2a and 2b show curves with respect to time of the secondaryvoltages U and U,,,,, and U,,,/2 and U ,,/2. The hatched areas whenthese voltages are switched through to the motors.

The curve of the alternating network current in shown in FIG. 2c.

The current commutation due to voltage reversal (secondary voltageschange from the negative to the positive half-wave) occurs at time twhereas the network current, because of inductances (motor, transformer,etc.), does not reverse its direction until Because in both converterunits l and II one bridge circuit is always being operated at full load(thyristors l2, and 12,, with diodes 9, and 9 the alternating net workcurrent follows the driving voltages U ,/2 and U,,,,/2. Because thechokes l4, and 14,, are magnetically in reverse-series connection(beginning of winding is marked by a spot) their effects cancel eachother out, so that the rise of the network current is not delayedunnecessarily. If the chokes l4, and 14,, were not in reverse-seriesconnection (i.e. separate) the network current would follow the pathindicated by the broken line, but this would result in an unfavourablepower factor (FIG. 2c).

At time for example, thyristor 7,, is fired, in addition to the fullyadvanced rectifier bridge with the conducting thyristor 12,,, so thatfrom this moment onwards the full secondary voltage U,,,, is applied tothe motor 13,,.

The resulting rise of the direct current is suitably delayed by choke14,, so that the harmonics imposed on the a.c. network are as weak aspossible.

Choke 14,, can become effective because this increased current is notcompensated by a corresponding current in choke 14,.

As soon as the now greater direct current flowing through motor 13,, hasbeen taken over completely by thyristor 7,,, choke 14,, ceases to carrycurrent. The direct current then flows away from the secondary winding6,, by way of thyristor 12,,, and back again by way of thryistor 7 'Whenthis state has been reached, thyristor 7, of rectifier arrangementl canbe fired in addition to the conducting thyristor 12,, at time t,,, forexample. Choke 14, then delays current commutation in the desired mannersince, as shown above, choke 14,, is dead. Because commutation occursmore slowly the rise of network current is also delayed accordingly.Once the commutation process is complete, choke 14, is also dead. Thedirect current flowing through motor 13, is then directed by way ofthyristors 7, and 12,.

When the direction of the driving secondary voltages U and U,,,,reverses at time t.,, thyristors l1, and 11,, are fired (phase anglefully advanced), whereupon current commutation again takes place. Thedelayed current zero at I (FIG. 20) is due solely to the inductances ofthe motors l3, and I3 and of the rectifier transformer; chokes 14, and14,, are still dead.

During the negative voltage half-wave the process described above isrepeated in the corresponding manner.

It must be emphasised that the thyristors of both con verter unitsoperating at part load must always fire at different moments, asotherwise the effects of the chokes 14, and 14,, would always canceleach other out.

Here advantage is taken of the fact that when a trac tion vehicleaccelerates, the rear drive wheels, when viewed in the direction oftravel, are always pressed harder against the rails, and so can be usedto transmit greater torque. In the example in FIG. 1, motor 13,, isallocated to the rear drive wheels; it thus receives more power thanmotor 13,.

The rectifier arrangements l and II shown in FIG. 1 are considered astwo-stage because the maximum d.c. output voltage is variable in twocoarse steps.

Multiple-stage rectifier arrangements are also possible, however.

In FIG. 3 a primary winding 15 of a transformer is shown connected tothe single-phase a.c. network, on the one hand by way of a currentcollector 2 and a conductor wire 3, and on the other by (at least) onewheel 4 and a rail 5.

The motors supplied by the converter units l and II .are notspecifically shown.

Again converter unit I is described as being representative of bothunits.

The secondary winding 16, of the transformer is provided with tappingsafter one-fourth and one-half of its total number of turns. The centraltapping is here connected to the link joining diodes 22, and 23, whichrepresent the uncontrolled half of three semicontrolled converter bridgecirciuts connected in series on the a.c. side. The controlled halvescomprise thy- I'iStOrS 18 /19, and

In order to vary the d.c. output voltage continuously, only thyristors18,, for example, (for one voltage halfwave) and 19, (for the other) arealways operated at part load, while coarse graduation if effected bymeans of the other bridge circuits.

Each of the lines leaving secondary windings 16, and 16,, afterone-fourth of the total turns contains a choke 30, and 30 respectively,and the two chokes are magnetically in reverse-series connection.

The operating principle of the reverse-seriesconnected chokes 30, and30,, is the same as that of chokes 14, and 14,, described above (FIG.1).

The semi-controlled bridge circuits supplied by secondary winding 16,provide four coarse graduations (the same is also true, of course, ofsecondary winding :1)

The number of steps is doubled by connecting a further semi-controlledconverter bridge in series on the d.c. side to the threeseries-connected converter bridge circuits on the a.c. side. 7

This additional semi-controlled converter bridge consists of thyristors26, and 27, and diodes 28, and 29,, and is fed from another secondarywinding 17, of the transformer. Secondary windings l6, and 17, have thesame total number of turns.

Here it is also important that with converter units of otherwise thesame configuration the semi-controlled converter bridge circuitsoperated at part load must not be triggered simultaneously because thenthe chokes 30, and 30,, would be ineffective, which would result inpronounced harmonics in the a.c. network.

The number of stages of the converter units I and 11 shown in FIG. 1 canalso be doubled in the same manner as shown in FIG. 3. I

The invention is not restricted to the diagrams shown in the drawing.Converter units in groups of four, six, etc. may also be employed, forexample, in which case the chokes should be allocated in pairs, or alltogether on one core. The converter units can also be supplied by way ofseparate transformers.

What we claim is:

l. A static converter arrangement comprising at least two converterunits having substantially the same configuration, each said converterunit comprising at least two semi-controlled converter bridge circuitsfeeding a common load, each said bridge circuit including a full wavecontrollable rectifier bridge portion and an uncontrollable rectifierbridge portion common to said two bridge circuits, a transformer forsupplying alternating current to be rectified by said converter units,said transformer including a primary winding connectible to a source ofalternating current and a secondary winding supplying each of saidconverter units, the opposite ends of said secondary windings beingconnected respectively to the controllable rectifier portions of saidbridge circuits and a choke located in a circuit extending through aportion of said secondary winding and from a center tap thereon to thecommon uncontrollable rectifier portion of said bridge circuits, saidcontrollable rectifier portions of said bridge circuits being operableseparately from respective portions of said secondary windingestablished by said tap or in series across the ends of said secondarywinding, and said chokes of said converter units being arranged magnetically in a reverse-series connection.

2. A static converter arrangement as defined in claim 1 wherein a singletap on said secondary winding for each converter unit is made to thecenter point thereof and said chokes of said converter units areconnected respectively to lines leading from said taps.

lines leading from said second taps.

1. A static converter arrangement comprising at least two converterunits having substantially the same configuration, each said converterunit comprising at least two semi-controlled converter bridge circuitsfeeding a common lOad, each said bridge circuit including a full wavecontrollable rectifier bridge portion and an uncontrollable rectifierbridge portion common to said two bridge circuits, a transformer forsupplying alternating current to be rectified by said converter units,said transformer including a primary winding connectible to a source ofalternating current and a secondary winding supplying each of saidconverter units, the opposite ends of said secondary windings beingconnected respectively to the controllable rectifier portions of saidbridge circuits and a choke located in a circuit extending through aportion of said secondary winding and from a center tap thereon to thecommon uncontrollable rectifier portion of said bridge circuits, saidcontrollable rectifier portions of said bridge circuits being operableseparately from respective portions of said secondary windingestablished by said tap or in series across the ends of said secondarywinding, and said chokes of said converter units being arrangedmagnetically in a reverse-series connection.
 2. A static converterarrangement as defined in claim 1 wherein a single tap on said secondarywinding for each converter unit is made to the center point thereof andsaid chokes of said converter units are connected respectively to linesleading from said taps.
 3. A static converter arrangement as defined inclaim 1 wherein each siad secondary winding includes a central tapconnected to said common uncontrollable rectifier portion and a secondtap midway between said central tap and the end of the winding, and saidchokes of said converter units are connected respectively to linesleading from said second taps.